This invention relates to functional fluids having improved viscosity characteristics.
Functional fluids have many applications. They are particularly useful as lubricants between moving mechanical parts and as force transmission fluids, such as for example, hydraulic fluids. In industrial uses of these functional fluids, the viscosity is of utmost importance, due to the fact that these products are commonly employed under wide ranges of temperature. To obtain satisfactory performance, it is recommended that the functional fluids possess a convenient viscosity temperature relation and a satisfactory flame resistance.
Many compounds and various compositions have already been suggested as functional fluids. Tricresyl phosphates have been used, but their viscosity index is negative. The viscosity index of a fluid is an indication of the viscosity variation of the fluid as a function of the temperature and the most valuable products have a positive index. A negative index for tricresyl phosphates indicates that these products are not useful throughout the temperature ranges normally encountered.
Moreover, the viscosity of many phosphoric acid esters is often too low for the conditions of use which are generally required. To obviate this drawback, phosphoric acid esters having a high molecular weight may be employed. U.S. Pat. No. 2,964,477 discloses such polyesters which may comprise a di-(hydroxy aryl)-alkylidene and a monoester of phosphoric of phosphonic acid. The viscosity characteristics at low temperatures are particularly unfavorable for such high molecular weight materials, however, and this is shown by the relatively low viscosity index obtained, which in some instances is negative. Another known means for overcoming this disadvantage is by incorporating viscosity-index improvers, generally polymers such as polymethacrylates, polyolefines and the like. However, the flame resistance is substantially decreased.